The present invention relates to a new, economical, very efficient process for generating chlorine dioxide.
Inasmuch as chloride dioxide is of considerable commercial importance in the field of pulp bleaching, water purification, fat bleaching, removal of phenols from industrial wastes, textile bleaching, and the like, it is very desirable to have a process by which it can be economically generated.
One means for the generation of chlorine dioxide is by way of reaction of a chlorate, a chloride, and sulfuric acid providing an acid normality of about 2 to 12. The reactions which occur are exemplified below, whereby, for the sake of illustration, the chlorate used is sodium chlorate and the chloride used is sodium chloride. EQU (1) NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +Na.sub.2 SO.sub.4 +H.sub.2 O EQU (2) NaClO.sub.3 +5NaCl+3H.sub.2 SO.sub.4 .fwdarw.3Cl.sub.2 +3Na.sub.2 SO.sub.4 +3H.sub.2 O
This technique for chlorine dioxide production is used on a commercial scale, with the reactants continuously being fed into a reaction vessel and the chlorine and chlorine dioxide produced continuously being removed from the reaction vessel with large quantities of sodium sulfate (saltcake) by-product also being generated for use in kraft paper mill processes.
Another means for the generation of chlorine dioxide is by the reaction of a chlorate with hydrochloric acid at an acid normality of about 0.05 to about 1. The reactions which occur are exemplified below, wherein, for the sake of illustration the chlorate used is sodium chlorate. EQU (1a) 2NaClO.sub.3 +4HCl.fwdarw.2ClO.sub.2 +Cl.sub.2 +2NaCl+2H.sub.2 O EQU (2a) NaClO.sub.3 +6HCl.fwdarw.3Cl.sub.2 +NaCl+3H.sub.2 O.
The combined use of the reagents sodium chloride and sulfuric acid to convert sodium chlorate to chlorine dioxide has generally been equated in the prior art with the use of hydrochloric acid to convert sodium chlorate to chlorine dioxide (Sepall et al. U.S. Pat. No. 3,347,628, column 2, lines 39-52 and column 3, lines 1-2). In theory, sodium chloride and sulfuric acid react to produce hydrochloric acid in situ in accordance with the following equation: EQU (3) 2NaCl+H.sub.2 SO.sub.4 .fwdarw.2HCl+Na.sub.2 SO.sub.4
In actuality, however, an equilibrium exists between Na.sub.2 SO.sub.4 and sulfuric acid to produce NaHSO.sub.4 per the following reaction sequence EQU (4) SO.sub.4.sup.= +2H.sup.+ .revreaction.2HSO.sub.4.sup.- EQU (5) NaCl+H.sub.2 SO.sub.4 .fwdarw.HCl+NaHSO.sub.4
Thus, though the acidity range generally considered applicable in chlorine dioxide generation is about 2 to 12 normal in sulfuric acid, when sulfuric acid is employed as the strong acid, at temperatures above about 30 degrees centigrade, one will recover the neutral sodium sulfate (Na.sub.2 SO.sub.4) if the acidity of the reaction solution is maintained between about 2-4.8 normal (Winfield et al., U.S. Pat. No. 3,864,456). The production of sodium sulfate and sodium bisulfate occurs at higher acid concentrations.
The acidity range generally considered most desirable in chlorine dioxide generation in a hydrochloric acid system (mode) is about 0.05 to about 1.0 without a catalyst. Beyond about 1.0 normality the chlorine production increases at the expense of chlorine dioxide production so as to render the process inefficient (Canadian Patent 956,784 to Winfield). The use of catalysts, however, extends the cavity range which might be efficiently utilized up to about 1.9 normal (British Pat. No. 1,347,740 to Partridge et al).
Thus, heretofore to operate either a sulfuric acid-sodium sulfate system or a hydrochloric acid-sodium chloride system in a single vessel chlorine dioxide reactor-crystallizer-evaporator it was necessary to operate at two widely different acidity levels. In changing from one acid system to the other using the same reactor-crystallizer it was necessary to evacuate the first generator liquor to a holding tank and then refill the generator with the second liquor before continuing generation of chlorine dioxide.
It has now been discovered that if chlorine dioxide generator liquor is saturated with solutions of both alkali metal sulfate and alkali metal chloride it is permissable to operate under the same reaction conditions regardless of whether sulfuric acid or hydrochloric acid etc., is being used in the same generator. Thus, depending on what the particular salt by-product needs of a paper mill are at a given time, and the ready availability of acid, sulfuric or hydrochloric, it is now possible to operate under the same conditions at the same acid normality (about 2 to about 11) by simply introducing one acid or the other to the generator. This more flexible process eliminates required evacuation of the generator in switching from one system or mode to the other, and furthermore, offers a more satisfactory means of achieving balanced inventories of both sodium sulfate saltcake and sodium chloride by-products required in pulp bleaching processes and in producing sodium chlorate.
Accordingly, it is the principal object of the present invention to provide a more flexible process for production of chlorine dioxide and chlorine wherein various mineral acids may be used interchangeably in the same reactor without modification of operating conditions or changing generating liquors.
It is a further object of the present invention to provide a versatile process for generating chlorine dioxide and chlorine which enables greater control over salt by-product inventories.
A still further object of the instant invention is to provide a versatile high acidity hydrochloric acid system which will also permit highly efficient production of both chlorine dioxide and chlorine in a single vessel type generator-crystallizer-evaporator.
These and other objects, features and advantages will become more apparent from a reading of the following summary and more detailed description of the invention.